Refactoring the name "models" for the cert cache

I know this isn't new, but the name "models" for the cert cache is confusing. A model could be anything.

The best solution would be refactoring it to something like self.cert_models or even self.cert_cache, in my opionion.

Alternatively, a quick fix would be to add to this docstring and the constructor docstring that 'models' are CertModel objects (and maybe also what that means). Because now it just says "model cache" which didn't really help me figure out what this is.